1. Field of the Invention
This invention relates to a protection device used for assuring a safe operation of a rechargeable battery, such as a secondary cell or the like, and a battery cell provided with this protection device.
2. Description of the Related Art
When a charging is further continued even after the charging is completed in a rechargeable battery cell or when a charging is carried out with a higher level of current than that of a predetermined current, it sometimes occurs that an abnormal state is generated in the battery cell to produce gas within the battery cell, either an air pressure or a temperature within the battery cell is increased and extrusion of electrolyte caused by expansion or cracks of the battery cell corrode equipment assembled with the battery cell. In addition, a continuation of this use of the battery cell under such a condition even though corrosion may not occur causes a possibility in which the battery cell is bulged out, a crack or a breakage is generated in the battery cell and so the use of the battery cell in which a certain abnormal state is produced should be stopped immediately.
In view of the foregoing, a rechargeable battery cell is provided with various kinds of safety countermeasures such as a safety valve arranged in it to discharge gas in the battery cell in response to an increased air pressure within the battery cell or alternatively means arranged out of the battery cell for sensing the increased temperature and preventing an electric current from flowing to an electrical circuit to which the battery cell is connected.
However, in the case that the safety valve was required to be installed in the battery cell, a piping or the like in the battery cell had to be considered and this became a restriction on designing the battery cell or in the case of the type in which an increasing of temperature within the battery cell was detected, there was a possibility in which the battery cell was used again when the temperature was decreased as time elapsed.